Method of forming a flexible graphite sheet with decreased anisotropy

ABSTRACT

Flexible graphite sheet is made by compressing a mixture of relatively large particles of intercalated, exfoliated, expanded natural graphite with smaller particles of intercalated, exfoliated expanded, expanded particles of natural graphite. The resulting sheet of flexible graphite exhibits increased electrical conductivity through the thickness (&#34;c&#34; direction) of the sheet and improved sealability perpendicular to the &#34;c&#34; direction.

FIELD OF THE INVENTION

This invention relates to a method of making flexible graphite sheethaving reduced anisotropy, with respect to electrical resistivity, toprovide increased electrical conductivity through the sheet thickness,with respect to sealability, to provide improved sealability (lessleakage) perpendicular to the thickness of the flexible graphite sheet.

BACKGROUND OF THE INVENTION

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as basal planes, are linked or bonded together and groupsthereof are arranged in crystallites. Highly ordered graphites consistof crystallites of considerable size: the crystallites being highlyaligned or oriented with respect to each other and having well orderedcarbon layers. In other words, highly ordered graphites have a highdegree of preferred crystallite orientation. It should be noted thatgraphites possess anisotropic structures and thus exhibit or possessmany properties which are highly directional. Briefly, graphites may becharacterized as laminated structures of carbon, that is, structuresconsisting of superposed layers or laminae of carbon atoms joinedtogether by weak van der Waals forces. In considering the graphitestructure, two axes or directions are usually noted, to wit, the "c"axis or direction and the "a" axes or directions. For simplicity, the"c" axis or direction may be considered as the direction perpendicularto the carbon layers. The "a" axes or directions may be considered asthe directions parallel to the carbon layers or the directionsperpendicular to the "c" direction. Natural graphites possess a highdegree of orientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the "c"direction and thus form an expanded or intumesced graphite structure inwhich the laminar character is substantially retained.

Natural graphite flake which has been greatly expanded and moreparticularly expanded so as to have a final thickness or "c" directiondimension which is at least 80 or more times the original "c" directiondimension can be formed without the use of a binder into cohesive orintegrated sheets, e.g. webs, papers, strips, tapes, or the like. Theformation of graphite particles which have been expanded to have a finalthickness or "c" dimension which is at least 80 times the original "c"direction dimension into integrated sheets without the use of anybinding material is believed to be possible due to the excellentmechanical interlocking, or cohesion which is achieved between thevoluminously expanded graphite particles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy, e.g. with respect toelectrical and thermal properties. Sheet material can be produced whichhas excellent flexibility, good strength and a high degree oforientation.

Briefly, the process of producing flexible, binderless graphite sheetmaterial, e.g. web, paper, strip, tape, foil, mat, or the like,comprises compressing or compacting under a predetermined load and inthe absence of a binder, expanded graphite particles which have a "c"direction dimension which is at least 80 times that of the originalparticles so as to form a substantially flat, flexible, integratedgraphite sheet. The expanded graphite particles which generally areworm-like or vermiform in appearance, once compressed, will maintain thecompression set. The density and thickness of the sheet material can bevaried by controlling the degree of compression. The density of thesheet material can be within the range of from about 5 pounds per cubicfoot to about 125 pounds per cubic foot. The flexible graphite sheetmaterial exhibits an appreciable degree of anisotropy, e.g. as regardselectrical resistivity, with the degree of anisotropy increasing uponroll pressing of the sheet material to increased density. In rollpressed anisotropic sheet material, the thickness, i.e. the directionperpendicular to the sheet surface comprises the "c" direction and thedirections ranging along the length and width, i.e. along or parallel tothe surfaces comprises the "a" directions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning view electron microscope (SEM) at an originalmagnification of 100X showing natural graphite flake sized in the rangeof 20×50 mesh; and

FIG. 2 is a scanning electron microscope (SEM) view at an originalmagnification of 100X showing natural graphite flake sized 50×100 mesh;

DETAILED DESCRIPTION OF THE INVENTION

Graphite is a crystalline form of carbon comprising atoms bonded in flatlayered planes with weaker bonds between the planes. By treatingparticles of graphite, such as natural graphite flake, with anintercalant of, e.g. a solution of sulfuric and nitric acid, the crystalstructure of the graphite reacts to form a compound of graphite and theintercalant. The treated particles of graphite are hereafter referred toas "particles of intercalated graphite". Upon exposure to hightemperature, the particles of intercalated graphite expand in dimensionas much as 80 or more times its original volume in an accordion-likefashion in the "c" direction, i.e. in the direction perpendicular to thecrystalline planes of the graphite. The exfoliated graphite particlesare vermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets which,unlike the original graphite flakes, can be formed and cut into variousshapes.

A common method for manufacturing graphite foil from flexible graphiteis described by Shane et al in U.S. Pat. No. 3,404,061 the disclosure ofwhich is incorporated herein by reference. In the typical practice ofthe Shane et al method, natural graphite flakes are intercalated bydispersing the flakes in a solution containing an oxidizing agent of,e.g. a mixture of nitric and sulfuric acid. The intercalation solutioncontains oxidizing and other intercalating agents known in the art.Examples include those containing oxidizing agents and oxidizingmixtures, such as solutions containing nitric acid, potassium chlorate,chromic acid, potassium permanganate, potassium chromate, potassiumdichromate, perchloric acid, and the like, or mixtures, such as forexample, concentrated nitric acid and chlorate, chromic acid andphosphoric acid, sulfuric acid and nitric acid, or mixtures of a strongorganic acid, e.g. trifluoroacetic acid, and a strong oxidizing agentsoluble in the organic acid.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solutions maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

After the flakes are intercalated, any excess solution is drained fromthe flakes. The quantity of intercalation solution retained on theflakes after draining may range from 20 to 150 parts of solution byweight per 100 parts by weight of graphite flakes (pph) and moretypically about 50 to 120 pph. Alternatively, the quantity of theintercalation solution may be limited to between 10 to 50 parts ofsolution per hundred parts of graphite by weight (pph) which permits thewashing step to be eliminated as taught and described in U.S. Pat. No.4,895,713 the disclosure of which is also herein incorporated byreference. The intercalated graphite flakes are exfoliated into flexiblegraphite by exposing them to a flame for only a few seconds attemperature greater than 700° C., more typically 1000° C. or higher. Theexfoliated graphite particles, or worms, are then compressed andsubsequently roll pressed into a densely compressed flexible graphitefoil sheet of desired density and thickness and substantially increasedanisotropy with respect to electrical resistivity and other physicalproperties. Suitable exfoliation methods and methods for compressing theexfoliated graphite particles into thin foils are disclosed in theaforementioned U.S. Pat. No. 3,404,061 to Shane et al. It isconventional to compress the exfoliated worms in stages with the productof the first or early stages of compression referred to in the art as"flexible graphite mat" having a density of about 3 to 10 lbs/ft⁰.3 anda thickness of from 0.1 to 1 inch. The flexible graphite mat is thenfurther compressed by roll pressing into a standard density sheet orfoil of preselected thickness. A flexible graphite mat may be thuscompressed by roll pressing into a thin sheet or foil of between 2-180mils in thickness with a density approaching theoretical density,although a density of about 70 lbs./ft.³ is acceptable for mostapplications and suitably 10 to 100 lbs/ft³.

In a particular embodiment of the present invention, a first batch ofnatural graphite flake particles, i.e. naturally occurring graphiteflake, as shown in FIG. 1 (original magnification 100X), at least 80% byweight sized 20×50 mesh (through 20 mesh on 50 mesh), are treated bydispersing the naturally occurring flakes in an intercalating solutionsuch as above-described. After the flakes of the first batch areintercalated, excess solution is drained from the flakes which are thenwashed with water and dried. A second batch of smaller sized naturalgraphite flakes as shown in FIG. 2 (original magnification 100X), sizedat least 80% by weight 50 by 100 mesh (through 50 mesh on 100 mesh), istreated with an intercalating solution in the same manner as the firstbatch and similarly water-washed and dried. These unexfoliatedintercalated natural graphite flakes, at least 80% by weight 50 by 100mesh, are mixed and blended with the unexfoliated particles of the firstbatch to provide from about 25% to 75% by weight of the smaller sizedunexfoliated intercalated natural graphite flake in the blended mixture.The unexfoliated intercalated natural graphite flake particles arereadily mixed to provide a substantially uniform blend of unexfoliated,unexpanded flake particles. This can be achieved, for example, byspreading the finer, unexfoliated natural graphite particles over a bedof the larger unexfoliated natural graphite particles which arepositioned on a vibrating table.

The mixture of dried flakes is exposed to a flame for only a few secondsand the intercalated flake particles expand, i.e. exfoliate, intovermicular, worm-like particles which are about 80 to 1000 times thevolume of the initial dried intercalated flakes.

The use of more than 80% by weight of the smaller size particles hasbeen found to result in a fragile sheet product which does not have goodtensile strength; the use of amounts of the smaller sized particles ofless than 25% by weight does not significantly affect the anisotropy ofthe resulting flexible graphite sheet as regards electrical resistivity.

The mixture of large and small exfoliated graphite particles isroll-pressed into sheet or foil typically 0.002 to 0.180 inch mm thickand having a density of at least 10 lbs./ft.³. The resultant sheet, orfoil, is characterized by having reduced electrical resistivity, i.e.increased electrical conductivity, across the thickness ("c" direction)of the sheet or foil. As the proportional amount of the smaller sizeparticles (50×100 mesh) is increased, the electrical conductivity in the"c" direction of the sheet or foil is increased which is important whenthe sheet or foil is used as a component of a fuel cell electrode asdescribed in U.S. Pat. No. 5,300,370 with reference to "GRAFOIL" whichis the trade designation for flexible graphite products of UCAR CarbonCompany Inc.

EXAMPLE I (PRIOR ART)

Natural graphite flake, sized 80% by weight 20×50 mesh, (FIG. 1) wastreated in a mixture of sulfuric (90 wt. %) and nitric acid (10 wt. %).The thus treated intercalated natural occurring flake was water washedand dried to about 1% by weight water. A portion of the treated,intercalated heat expandable natural graphite flake was introduced intoa furnace at 2500° F. to obtain rapid expansion of the flake into onepound of vermicular, worm shaped particles having a volume of about 325times that of the unexpanded intercalated flake.

The worm shaped heat expanded, intercalated graphite flake was rolledinto a sheet about 0.030 inch thick and 24 inches in width and a densityof 45 lbs./ft.³. Samples of the 0.030 inch thick sheet had an electricalresistivity of 10,500 μm Ωm (micro ohm meters) in the direction of thethickness of the sheet ("c" direction).

EXAMPLE II (This Invention)

A first batch of natural graphite flake, sized 80% by weight 20×50 mesh,(FIG. 1) was treated in a mixture of sulfuric (90 wt. %) and nitric acid(10 wt. %). The thus treated, intercalated natural graphite flake waswater washed and dried to about 1% by weight water.

A second batch of smaller sized natural graphite flake, sized 80% byweight 50×100 mesh (FIG. 2) was treated in a mixture of sulfuric andnitric acid and water washed in the same manner as the first batch oflarger sized natural graphite to obtain intercalated, unexpanded heatexpandable natural graphite flake.

Different amounts of the intercalated, unexpanded, heat expandablenatural graphite flake of the smaller particle sized second batchmaterial was blended with one (1) pound of the intercalated, unexpanded,heat expandable natural graphite particles of the first batch to provideblended mixtures containing from about 25 to 75 by weight of the smallersized unexpanded, intercalated natural graphite flake.

The mixture of dried flakes are exposed to a flame for only a fewseconds and the intercalated flake particles expand, i.e. exfoliate,into vermicular, worm-like particles which are about 80 to 1000 timesthe volume of the initial dried intercalated flakes.

The mixtures of worm shaped, heat expanded, natural graphite particleswere roll pressed into a sheet about 0.030 inch thick and 24 inches inwidth and 45 lbs/ft.³.

Samples (2.5 inches diameter) of the sheet from this Example II weretested for electrical resistivity as compared to Example I, as shown inthe following Table. Also, gasket-shaped samples of 80 lbs./ft.³density, in the form of rings (50 mm ID×90 mm OD, as specified in DIN28090-1) 0.030 inch thick were tested for sealability with the resultsshown in the Table:

    ______________________________________                                        % by Weight of Smaller                                                        (50 × 100 mesh)      Sealability                                        Starting Particles                                                                         Electrical Resistivity                                                                      (DIN 28090-1) ml/min                               ______________________________________                                        0            10,500 m Ωm                                                                           0.77                                               25            5,200 m Ωm                                                75            2,800 m Ωm                                                                           0.48                                               ______________________________________                                    

As shown above, the electrical resistivity decreases with addition ofthe 25%, about halved the resistivity, 75% addition had about one fourththe resistivity while substantially maintaining the handlability(strength and flexibility) necessary to utilize the materialcommercially. Electrical resistivity was obtained using a Keithley 2001Multimeter and four probe gold plated platens at 225 psi pressure on thesamples.

* Mesh sizes used herein are United States screen series.

What is claimed is:
 1. Method for making flexible graphite sheet having decreased electrical resistivity through the thickness and improved sealability perpendicular to the thickness of the sheet ("c" direction) comprising:(i) providing a first batch of relatively large size natural graphite flake sized at least 80% by weight 20×50 mesh; (ii) providing a second batch of smaller sized natural graphite flake sized at least 80% by weight 50×80 mesh; (iii) blending said first batch and said second batch to provide a blended mixture containing from about 25 to 75 percent by weight of the smaller sized natural graphite flake of said second batch; (iv) treating said blended mixture with an intercalating solution to obtain a heat expandable, intercalated graphite flake mixture; (v) exposing the intercalated natural graphite flake mixture of step (iii) to an elevated temperature to exfoliate said intercalated natural graphite flake into a mixture of relatively large size and smaller size expanded vermicular worm shaped particles of graphite; and (vi) passing the blended mixture of step (v) through pressure rolls to form a coherent, roll pressed, compressed sheet formed of said blended mixture of pre-determined thickness, the electrical resistivity through the thickness of the compressed sheet decreasing with increasing amounts in the blended mixture of step (iii) of the smaller sized natural graphite flake of step (ii).
 2. Method in accordance with claim 1 wherein the mixture of step (vi) is roll pressed to form a sheet having a thickness of from 2 to 180 mils and a density of at least 10 to 100 lbs./ft.³. 